1. Technical Field of the Invention
The present invention relates to extremely excellent, frozen dough-resistant, practical baker""s yeast.
Conventional frozen dough-resistant baker""s yeast has heretofore been known, over which the frozen dough-resistant, practical baker""s yeast of the invention is significantly excellent.
Frozen dough as produced through the process of preparing dough with the frozen dough-resistant baker""s yeast of the invention followed by incubating and freezing it is resistant to long-term frozen storage of 2 weeks or longer, from which is produced good bread. The long-term stored, frozen dough gives, when thawed and baked, better bread than that from the frozen dough as prepared with the conventional frozen dough-resistant baker""s yeast and stored long. Specifically, in the invention in which the NTH1 gene in practical baker""s yeast having various excellent characteristics but not having resistance to frozen dough is inactivated, it has become possible to make the practical baker""s yeast have frozen dough resistance that is comparable to or higher than that of ordinary commercially-available, frozen dough-resistant baker""s yeast.
Therefore, the frozen dough-resistant, practical baker""s yeast of the invention greatly contributes to developments in the frozen dough industry.
2. Prior Art
(Accumulation of Trehalose)
Regarding the frozen dough resistance of yeast, a technique of gene manipulation to ensure the accumulation of trehalose in yeast was reported by Helmut Holzer et al. of the Freiburg University (see J.B.C., Vol. 268, No. 7, 1993).
In their report, the NTH1 gene (neutral trehalase gene) of yeast was cloned, and then URA3 (uridylic acid synthetase gene) was introduced into a-type and xcex1-type NTH1 genes to thereby disrupt the NTH1 gene in the yeast. Through their technique reported, they confirmed the increase in the accumulation of trehalose in the yeast with no decomposition of trehalose therein.
On the other hand, Johan M. Thevelein et al. inserted LEU2 into the NTH1 gene of xcex1-type and a/xcex1-type yeasts to thereby disrupt the NTH1 gene therein, and confirmed the accumulation of trehalose in the resulting yeasts (see Applied and Environmental Microbiology, Vol. 61, No. 1, January 1995, pp. 105-115).
However, they concluded that their technique is ineffective in producing frozen dough-resistant baker""s yeast.
As above, it is known to disrupt the NTH1 gene of a-type, xcex1-type and a/xcex1-type yeasts with URA3 to thereby increase the amount of trehalose to be accumulated in those yeasts.
(Hybridization of Yeast)
In general, baker""s yeast includes haploids (a-type and xcex1-type), diploids (a/xcex1-type, a/a-type, a /xcex1-type), triploids (diploid x a-type or xcex1-type), tetraploids (diploid x diploid), etc. At present, in Japan, almost all commercially-available baker""s yeasts are a/xcex1-type diploids.
For obtaining excellent diploid baker""s yeast, two methods are known. One is to obtain a variety of mutants from original diploid yeast strain by spontaneous, or mitogen induced mutagenesis, and to screen them to select mutants with good properties; and the other is to mate haploid a-type yeast with good properties and a haploid xcex1-type yeasts with good properties respectively, and to screen the resulting diploid yeasts to select hybrids with good properties.
To mate them, an a-type yeast and an xcex1-type yeast of the same amount are mixed and cultivated together, whereupon in about 12 hours after conjugation of the two in which are formed hybrids. This technique is already known.
The conventional gene manipulation of disrupting the NTH1 gene (neutral trehalase gene) in yeast may produce the increase in the amount of trehalose to be accumulated in the resulting yeast, but frozen dough-resistant, practical baker""s yeast capable of finally giving delicious bread could not be obtained as yet. Given this situation, the object of the invention is to construct frozen dough-resistant, practical baker""s yeast capable of finally giving delicious bread, to produce excellent frozen dough, and to produce delicious bread by thawing, fermenting and baking the frozen dough.
Even though freezing-resistant yeast could be constructed through NTH1 gene disruption, frozen dough-resistant, practical baker""s yeast could not be obtained as yet. We, the present inventors desired to modify practical baker""s yeast having excellent properties but not having resistance to frozen dough into frozen dough-resistant, practical baker""s yeast still having its original excellent properties and additionally having frozen dough resistance that is comparable to or higher than that of ordinary, commercially-available freezing-resistant yeast. For this purpose, we analyzed in detail starting yeast strains, frozen dough and even final bread in various experiments and, as a result, have completed the invention.
The invention relates to a set of NTH1 gene-disrupted, haploid yeasts as produced through gene manipulation of disrupting the NTH1 gene in a set of haploid yeasts of which the original hybridized diploid is practical baker""s yeast.
The invention also relates to a diploid or higher polyploid, frozen dough-resistant, practical baker""s yeast as produced through mating with one or more NTH1 gene-disrupted, haploid yeasts produced through gene manipulation of disrupting the NTH1 gene in a haploid yeast of which the diploid is practical baker""s yeast. Where two or more yeasts are used in that mating, at least one of those is the NTH1 gene-disrupted, haploid yeast while the others may be yeasts with no gene disruption.
The invention further relates to frozen dough-resistant, practical baker""s yeast-containing, frozen dough, as produced by preparing dough with a diploid or higher polyploid, frozen dough-resistant, practical baker""s yeast that is produced through mating with one or more NTH1 gene-disrupted, haploid yeasts produced through gene manipulation of disrupting the NTH1 gene in a haploid yeast of which the diploid is practical baker""s yeast, then incubating it and thereafter freezing it. Optionally in the invention, the frozen dough is thawed, fermented and baked to give delicious bread.